Moscow: The Kremlin on Friday lashed out at the White House after it backed up an allegation from the US Treasury that President Vladimir Putin is corrupt.
US Treasury acting under secretary for terrorism and financial intelligence Adam Szubin said in a BBC documentary aired Monday that Putin was a "picture of corruption".
White House spokesman Josh Earnest on Thursday backed up that line, saying that the Treasury's assessment "best reflects the administration view."
The Kremlin has already dismissed the US Treasury claim that it said amounted to an "official accusation", but ratcheted up the rhetoric after the White House got involved.
"We consider this statement outrageous and offensive," spokesman Dmitry Peskov told journalists.
"We really need further explanation because such a statement is absolutely unprecedented."
Ties between Moscow and Washington have plunged to their lowest point since the Cold War over Russia's meddling in Ukraine.
The two sides, however, are currently engaged in an international peace push on the conflict in Syria, although they support different sides in the civil war.
Peskov accused Washington of firing the starting gun on attempts to discredit Putin ahead of Russia's next presidential elections in 2018, even though he insisted Putin has not yet decided to run.
Russian authorities have repeatedly accused the West of plotting to overthrow Putin, but critics insist an elite around the strongman is whipping up public fears as they cement their grip over the country's vast wealth.